fix(expression): return a parse error for non-string type nodes in expression JSON

[LuciferYang]

What

Expression JSON deserialization in src/iceberg/expression/json_serde.cc called json[kType].get<std::string>() (and one json[kTerm].get<std::string>()) guarded only by is_object() and contains(kType). A non-string "type"/"term" made nlohmann throw json::type_error.302, which escaped these Result-returning functions (the expression parse chain has no try/catch) and terminated the caller instead of returning an error. This is the same failure mode as the merged #857.

It is reachable, not test-only. Table-metadata parsing hits the type-aware LiteralFromJson through FieldFromJson's initial-default/write-default handling, and REST responses hit the expression parsers through the scan-metrics report filter and the residual/partition/plan filters, all via ICEBERG_ASSIGN_OR_RAISE, which forwards a Result error but not a thrown exception.

Closes #979.

How

Each of the six get<std::string>() sites now checks is_string() first and returns JsonParseError on a non-string node, mirroring the sibling OperationTypeFromJson. Valid string input is unaffected: the added is_string() sits in a short-circuit &&, so a well-formed node evaluates exactly as before, and the guard only rejects input that previously threw.

Testing

NonStringTypeIsParseError in expression_json_test.cc covers all six guards: a non-string "type" at the top level (ExpressionFromJson), on an and/or node, on a predicate's term node (routed through the transform-term check and then the named-reference parser), a non-string reference "term", and a non-string "type" on both the untyped and the type-aware LiteralFromJson overloads. Each case returns kJsonParseError; without the fix the corresponding input throws json::type_error.302 out of the Result-returning function. Removing any single guard makes one of these cases throw, so no guard is left unpinned.

[Copilot]

Copilot review overview

🟢 Approval recommended

The guards comprehensively address the exception paths with focused regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Prevents malformed expression JSON from throwing exceptions outside the Result contract.

Changes:

• Validates "type" and "term" nodes before string conversion.
• Adds regression coverage for all six affected parsing paths.

File	Description
src/​iceberg/​expression/​json_serde.cc	Guards string conversions and returns parse errors.
src/​iceberg/​test/​expression_json_test.cc	Tests non-string discriminator handling.

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